Since a piezoelectric motor (Published Unexamined Japanese Patent Application No. S52-29192) directly converting flexible movement of a piezoelectric element into impelling force of a slider and a rotor was developed, many ultrasonic actuators generating bending wave of a stator (for example, refer to Published Unexamined Japanese Patent Application No. S59-96881, Published Unexamined Japanese Patent Application No. S60-174078) have been developed, mainly using flexible movement of the piezoelectric element pasted up on the stator. Some multi-degree-of-freedom ultrasonic motor rotating a spherical rotor by some stators (refer to Published Unexamined Japanese Patent Application No. S62-228392, Published Unexamined Japanese Patent Application No. H9-34409, Published Unexamined Japanese Patent Application No. H11-18459) also have been developed. The ultrasonic actuators, however, have three major problems. The first is the low conversion efficiency of energy because flexible movement of the piezoelectric element must convert into the bending wave after converting it into vibration in the direction of polarization. The second is the low transfer efficiency of the friction power because a contact area between a stator and a slider or a rotor is small. The third is a complex drive device because sine waves with at least two phases are required for driving the ultrasonic actuators. Although an ultrasonic actuator possible to be drived by a single-phase wave is also developed, because of a good structure of a stator (a principle of operation of ultrasonic actuator, [online]. Seiko Instruments Inc., 2000. [retrieved on 2000 Jul. 30]. Retrieved from the Internet: <URL:http://www.sii.co.jp/wd/new_page—4.htm>), the stator must be processed according to arrangement of piezoelectric elements. Moreover, this actuator is not suitable for applications where high torque is desired, because a contact area between a stator and a slider or a rotor is extremely small.
In addition, as a method converting flexible movement of a piezoelectric element into linear movement, piezoelectric impact mechanism directly generating friction power from the flexible movement of the piezoelectric element (for example, refer to Higuchi, Watanabe, Kudoh, “Precise Positioner Utilizing Rapid Deformations of a Piezoelectric Element”, JSPE, Vol. 54, No. 11, pp.2107-2112, 1988; Higuchi, Yamagata, “Precise Positioning Mechanism Utilizing Rapid Deformations of Piezoelectric Elements (2nd Report) —Motion Characteristics with Enhanced Friction—”, JSPE, Vol.58, No.10, pp.1759-1764, 1992) has also been developed. The piezoelectric impact mechanism can avoid the above third problem because it can move an object to any of two directions by applying a single-phase saw-tooth wave to the piezoelectric element. Furthermore, since the piezoelectric impact mechanism employs the flexible movement of the piezoelectric element as impelling force of a moving object, it is useful for solving the above first problem, but it has three more problems as follows: First, the piezoelectric element must be the hard quality of the cylindrical material because it must connect two objects and flip them off according to the flexible movement. Therefore, a material usable practically for the piezoelectric element is restricted to piezoelectric ceramics like PZT (Lead Zirconate Titanate). Second, many piezoelectric elements must be stacked, and moreover high voltage must be applied to them because they must vibrate two heavy objects, maintaining their intensity. Third, stress generated by the piezoelectric elements is applied to two objects in the opposite directions, respectively, because the piezoelectric elements move with the two objects, repeating their flexible movement. As a result, the stress of both objects are offset, and then the conversion efficiency of energy becomes low. Thus, the piezoelectric impact mechanism can not be used instead of the conventional actuators, although it is suitable for application desiring minute movement like a micromachine.
Now, as understanding from the piezoelectric impact mechanism, some piezoelectric elements can generate big stress collectively, by stacking them. Moreover, the stress can flip off an object in a case of using a single-phase saw-tooth wave. The hard piezoelectric elements are used to flip off the object in the piezoelectric impact mechanism, but the piezoelectric elements can be soft if they can flip off the object without applying their stress to the object directly. That is, any of a plate and a thin film are sufficient as the piezoelectric elements. As a result, voltage applied to the piezoelectric elements can be made low. Moreover, the stress can be converted into impelling force of the object efficiently, because the stress can move the object in one direction if the piezoelectric elements do not move with the object.
Considering these facts, suppose that a plurality of piezoelectric elements formed in the plate or the thin film are arranged as they overlap, the end of each piezoelectric element is fixed on a foundation, and a saw-tooth wave is applied to them. An object can be flipped off by friction power generated between the piezoelectric elements and itself. Thus, the above first and third problems come to be solved. In addition, the friction between the piezoelectric elements and the object increase by attaching unevenness to contact surface between them. Thus, the above second problem comes to be solved.
By the way, many kinds of small actuators have been recently developed besides an ultrasonic motor. A vibrator is listed as a major application using these actuators (for example, refer to Published Unexamined Japanese Patent Application No. H5-168195, Published Unexamined Japanese Patent Application No. H10-336983). However, when vibrators using a coil are carried in a cellular phone with the remarkable formation of the small lightweight, and so on, they have some problems because of their weight, thickness, and moreover generating lines of magnetic force. For another example, a table moving an object has been developed, where some actuators using a piezoelectric element and a thermoelectric element are arranged on a plane (for example, refer to Karl, F. Böhringer, Bruce R. Donald, Noel C. MacDonald, “Computational methods for design and control of MEMS Micromanipulator Arrays”, IEEE Computational Science & Engineering, pp. 17-29, January-March, 1997). However, the piezoelectric element and the thermoelectric element can not generate a force enough for moving a small object around us because they are researched as a micromachine.
Considering these facts, a small lightweight vibrator comes to be made by manufacturing an actuator using a thin and light piezoelectric element. In addition, some actuators, in which many piezoelectric elements are overlapping, comes to move a small object around us, by arranging them on a plate.
In the present invention described in claims, flexible movement of a plurality of piezoelectric elements are directly converted into movement of a stator and a rotor, by arranging the piezoelectric elements formed in the plate or the thin film as they overlap, and moreover, fixing the end of each piezoelectric element on a foundation. In addition, unevenness which makes to increase friction power between the piezoelectric element and the stator or the rotor is developed. Furthermore, a drive device is simplified by applying a saw-tooth wave to the piezoelectric element.